so we don't accidentally mail out events people shouldn't see

Say who the messages are from if there's no room name, otherwise it's a bit nonsensical

Add public facing base url to the server so synapse knows what URL to use when converting mxc to http urls for use in emails

Include name of the person we're sending to and add summary text at the top giving an overview of what's happened.

So names of people in a room are given in order

Doesn't get character enocind right yet but makes it not error.

Mostly WIP porting the room name calculation logic from the web client so our room names in the email mirror the clients.

Also pep8 fixes

Copy the stuff over from http pusher that prevents multiple instances of process running at once and sets up logging and measure blocks.

Mostly logic of when to send an email

Split out BaseMediaResource into MediaRepository

This is so that a single MediaRepository can be shared across all
resources, rather than having a "copy" per resource.

In particular this allows us to guard against both the thumbnail and
download resource triggering a download of remote content at the same
time.

Create log context in Measure if one doesn't exist

Lower timeout for make_membership_event

Simplify _check_password

fix cyrillic URL previews by hardcoding all page decoding to UTF-8 for now, rather than relying on lxml's heuristics which seem to get it wrong

Calls to make_membership_event are done in response to client requests,
and so should not be retried over long timeframes.

Use SynapseError 504 for Timeout errors

Fix login to error for nonexistent users

Fixes SYN-680

Make v2_alpha reg follow the AS API specification

The spec is clear the key should be 'user' not 'username' and this is indeed
the case for v1. This is not true for v2_alpha though, which is what this
commit is fixing.